1. Field of the Invention
The invention relates to optical apparatus for detecting magnetic fields where such detection is based on magnetic rotations to a polarized light beam.
2. Description Relative to the Prior Art
It is well known to detect magnetic fields based on Kerr or Faraday effect rotations to a polarized inspection beam. Unfortunately, such optical detection of magnetic fields, generally recordings, tends to yield an undesirably low signal-to-noise ratio. This is because of the small magnetic rotation angles which are produced and the numerous possibilities for introducing noise when attempting to detect such angles.
In the prior art, regarding optical detection of magnetic recordings, various approaches have been taken to reduce noise levels. It is known, for example, to utilize signal differencing techniques for cancelling certain types of noise. With such techniques the polarized inspection beam is split into two parts and two slightly displaced beam analyzers are employed for detecting rotation angles. By differencing the analyzer outputs, certain of the noise components are caused to cancel whereas a signal representative of beam rotation is retained.
It is also known to oscillate the polarizaton direction of the inspection beam to provide a reference frequency for use in detecting magnetic rotations. U.S. Pat. No. 3,947,890 describes such a technique wherein the degree of assymetry introduced in an output waveform by magnetic rotations serves to indicate the nature of magnetic records. In a preferred implementation of that technique, frequencies around the beam oscillation frequency and the second harmonic thereof are isolated for detecting magnetic rotations.
Notwithstanding these noise reduction techniques it would be desirable to have other approaches for increasing signal-to-noise ratios in an optical detection apparatus.